The present invention relates to the manufacture of integrated circuitry and, more particularly, to a method of selectively etching an insulating layer, such as a layer of silicon dioxide, with a plasma discharge.
Integrated circuitry is typically manufactured by forming a multiple number of the desired circuits on a semiconductive substrate, called a wafer. The wafer is then cut-up (diced) to provide the individual circuits. It is common during the manufacture, to use etching to remove selected portions of one material from another. For example, an insulating layer, such as a layer of silicon dioxide, is often applied to a whole surface of an electrically conductive or semiconductive material (the wafer substrate or a layer of material applied earlier in the processing operation). A portion of the insulating layer is then removed to provide a desired insulating layer configuration. This portion typically is removed by masking with, for example, photoresist, that portion which is not to be removed while maintaining exposed the portion to be removed. The exposed portion is then removed by subjecting the whole insulating layer and material therebeneath to an etching process.
There are several different etching processes, the selection of the one to be used in any particular situation being dependent upon several factors, including the operation stage, the material to be removed, the underlying material, availability for particular manufacturing processes, etc. One of such types is so-called plasma etching. In plasma etching, the exposed portion of the material to be removed is subjected to a plasma discharge of a gas selected to etch away the material it is desired be removed.
It is often quite important that the material to be etched or removed by plasma etching, be removed without damage to the underlying material. As a practical matter, etching will not be completed for all parts of a wafer containing integrated circuitry at the same time. The result is that there will be portions of the underlying material which will be subjected to the etching plasma before complete etching over the full wafer surface, of the material desired to be removed. Thus, in order to prevent damage to the underlying material, it is important that the plasma etching process be "selective", i.e., will continue to etch or remove the material desired to be removed without causing significant damage to any exposed portion of the material that is not to be removed.
Because of the problem of potential damage to an underlying material, some plasma etching is effected as a two stage operation. That is, the masked body to be subjected to plasma etching is subjected to a first plasma discharge of a gas which is designed to provide the majority of the desired etching. However, it is not necessary that such gas be selective, i.e., not deleteriously etch the underlying material, since little, if any, of such material will be exposed to the discharge. A second stage of plasma etching is then used. This stage must provide the selective etching, since it is during such stage that portions of the underlying material will be exposed.
It has been found that the use of a fluorocarbon as the active ingredient in a plasma discharge provides etching with the desired selectivity when the material to be removed is an insulating layer such as silicon oxide and the underlying layer is an electrically conductive or semiconductive material, such as monocrystalline or polycrystalline silicon. Typical fluorocarbons used are carbon tetrafluoride (CF.sub.4); hexafluoroethane (C.sub.2 F.sub.6); and octofluoropropane (C.sub.3 F.sub.8). Such a gas is a source of fluorine ions for the discharge-fluorine ions being quite reactive and providing a good etch rate. Such a gas is also a good source of the carbon believed to be necessary to cause the polymerization as discussed below thought to be responsible for good selectivity. In view of the necessity of such etching selectivity, the actual gas selection for a particular process is made with a view toward reducing the fluorine to carbon ratio. Care mut be taken, though, to provide a desired selectivity and yet avoid gross polymerization. Hydrogen is often added to the gas in small amounts to combine with the fluorine that is liberated and thereby reduce the amount of free fluorine in the plasma. Sometimes another fluorocarbon, such as CHF.sub.3, is provided as the source of hydrogen. An inert cooling or carrier gas, such as helium also is typically included.
With fluorocarbon chemistry, it is believed that etching of the underlying material by fluorine ions is inhibited by passivating the surface of such material with a polymer. Whether or not this is the phenomenon, it is known that a polymer (polytetrafluoroethylene) is formed by the plasma discharge of a fluorocarbon gas. The difficulty is that this polymer coats the exposed areas of the plasma chamber, i.e., the process chamber used to contain the plasma discharge. Such coating is quite dramatic. Even if care is taken to assure that gross polymerization rather than etching does not occur, the plasma chamber typically will be rendered useless by polymer coating, after processing of only a small number of integrated circuit wafers. The processing then must be stopped while the chamber is cleaned. This significantly affects material flow rate.